Automatic riveting machines can have various configurations, including a C-frame arrangement, such as shown in FIG. 1, a rectangular D-frame arrangement, or other machine arrangement with the part held in a horizontal plane and with the riveting axis in a generally vertical orientation or a gantry-type machine with the part vertical and the riveting axis horizontal. All of these machines use generally the same mechanism for carrying a rivet or bolt to a drilled opening in the workpiece, the carrying mechanism being positioned on the end of a ram and including spring-loaded gripping fingers for the fastener.
These machines use a CMC for control of rivet upset and logic. A CMC is both a logic controller and a motion controller, controlled by one processor or multiple processors connected together. The CMC controls upper and lower rams (or front and back rams) of a riveting machine and applies logic and timing to the motion control of the machine, as well as recognizing input and output information. Examples of CMCs include Delta Tau PMAC controller, Fanuc controllers and Siemens controllers.
A large ram force produced by an actuator is necessary to upset a rivet after it has been positioned in the workpiece opening or to drive a bolt into an interference fit in an opening in the workpiece. A rivet or bolt may in some cases not successfully initially enter the opening, because it is jammed between the ram die and the workpiece or turned sideways (laid down). In either case, when the riveting or interference force is applied to the misaligned fastener, the resulting damage to the workpiece can result in the entire workpiece being ruined, with a substantial monetary loss.
Accordingly, it is desirable that the manufacturing apparatus be able to automatically detect when a fastener (rivet or bolt) is not positioned properly in the workpiece prior to the application of the large ram force. One previous approach in solving this problem uses a camera to ensure proper insertion of the fastener. While this has been generally successful, it has limitations with respect to certain types of fastener misalignment and is insensitive to the case where the rivet is misaligned perpendicular to the view of the camera. Such a vision system also is expensive and has the further disadvantage of slowing down the fastening process, because the machine must actually stop during every cycle to perform a vision check.
In another previous approach, the push-away of the lower clamp portion of a riveting system is sensed. A lower clamp is held against the workpiece pneumatically in riveting operations. In a normal rivet cycle, the lower clamp is not pushed away but in the case of a rivet jam or a sideways, laid-down situation, the lower clamp is pushed away from the workpiece. While this technique is effective in reducing damage, it does not prevent it, since the clamp motion which is sensed has already resulted in at least some workpiece damage before the riveting force is interrupted. The lower clamp push-away technique cannot be used for bolts, however, because the lower clamp is not held pneumatically against the workpiece for bolt insertion. The full unexpected force can be detected by a load cell arrangement but catastrophic damage is done to the workpiece before the motion of the ram is stopped.
Accordingly, existing systems for preventing damage due to misaligned rivets and bolts are not completely satisfactory. It is important that all or virtually all instances of misalignment be quickly recognized and the fastener process interrupted prior to the application of fastening force and resulting damage to the workpiece.